1. Field of the Invention
The present invention relates to a starter for an internal combustion engine, and more particularly to a starter for an internal combustion engine, in which a rotation transmitting mechanism for transmitting the rotation of an input portion to a crank shaft is provided with a torque absorbing mechanism.
2. Description of Background Art
A kick type starter of an internal combustion engine mounted on a motorcycle is configured such that an actuated force of a kick pedal is converted into a kick shaft rotating force by a kick pedal shaft and the rotation of the kick shaft is transmitted to a crank shaft via a rotation transmitting mechanism. The rotation transmitting mechanism is composed of a kick drive gear, ratchet portion formed on a pinion gear, a pinion gear, gear train connected to a counter shaft and a main shaft of a transmission, and a primary reduction drive gear connected to the crank shaft.
Since the crank shaft is in a stopped state when the kick pedal is actuated, a large torque is applied to the gears of the rotation transmitting mechanism. Because of not only the excessively increased torque due to actuation of the kick pedal but also the occurrence of a compressive reaction upon upward movement of a piston during the kick operation or abnormality of an ignition timing, an excessive torque may be applied to the gears of the rotation transmitting mechanism during the starting operation. In the prior art rotation transmitting mechanism, the size and strength of each gear have been set in consideration of the above-described excessive torque applied to the gear.
A starter for starting an engine of a motorcycle, including a damper device for absorbing an excessive torque caused upon starting the engine, has been disclosed in Japanese Utility Model Publication No. Sho 63-16863. The starter is of a type using a starter motor, in which the rotation of the starter motor is transmitted to a crank shaft from a reduction gear through a starter, driven gear, characterized in that damper springs are contracted between a clutch outer and a clutch inner of an over-running clutch for absorbing an excessive torque caused upon starting the engine, on the other hand, a technique concerned with a drive shaft of a motorcycle and a gear press-fitted around the drive shaft has been disclosed, in Japanese Patent Publication No., Hei 4-16649. In this technique, one of contact surfaces (press-fit surfaces) of the drive shaft and the gear is subjected to a low temperature sulphurizing treatment for suitably setting a slip generating torque between both the members, thereby damping a reaction force from a tread or the like caused upon power transmission.
The starter described in Japanese Utility Model Publication No. Sho 63-16863, however, has a problem that since a plurality of the springs must be disposed between a plurality of projecting pieces of the clutch outer and a plurality of projecting pieces of the clutch inner in an annular shape as a whole, the structure of the damper is complicated and the size thereof is enlarged.
The technique described in Japanese Patent Publication No. Hei 4-16649 also has a problem. Since the driver shaft has a relatively small diameter for the portion subjected to the low temperature sulphurizing treatment being one of the press-fit surfaces of the driver shaft and the gear, a press-fit amount must be enlarged for obtaining a desired slip generating torque from the press-fitted portions of the drive shaft and the gear, with a result that the press-fitted portions tend to be made longer.
An object of the present invention is to provide a starter for an internal combustion engine, which is excellent in durability and has a compact structure, characterized in that a desired slip generating torque can be set with a small press-fit amount.
According to the present invention, there is provided a starter for an internal combustion engine including a rotation transmitting mechanism for transmitting the rotation of an input portion to a crank shaft, wherein the rotation transmitting mechanism is provided with a torque absorbing mechanism for absorbing an excessive torque caused by an excessive input from the input portion or a reversed input from the crank shaft, the starter being characterized in that the torque absorbing mechanism is provided on a supporting shaft and includes a gear set composed of an input gear and an output gear which are press-fitted to each other; and at least one of a press-fit surface of the input gear and a press-fit surface of the output gear is subjected to a low temperature sulphurizing treatment.
In accordance with the present invention, since at least one of the press-fit surfaces of the gear set is subjected to the low temperature sulphurizing treatment, the press-fit surfaces have a low friction coefficient and a good wear resistance. As a result, it is possible to obtain the starter including the rotation transmitting mechanism having the torque absorbing mechanism capable of stably keeping a slip generating torque for a long-period of time. Also, since the torque absorbing mechanism is configured by the gear set composed of the gears press-fitted to each other, the starter is made compact. This is advantageous in terms of layout of the starter. Further, since at least one of the press-fit surfaces of the input and output gears each of which has a diameter larger than that of the supporting shaft is subjected to the temperature sulphurizing treatment, a fastening force for setting a desired slip generating torque can be set at a value smaller than a fastening force obtained by press-fitting in the case where the torque absorbing mechanism is composed of the supporting shaft and a gear which is press-fitted around the supporting shaft. In other words, the press-fit amount in the case where the input and output gears, each of which has a diameter larger than that of the supporting shaft, are press-fitted to each other, maybe smaller than the press-fit amount in the case where the gear is press-fitted around the supporting shaft.
According to the present invention, because of the fact that the fastening force is small in addition to the fact that the press-fit surface is subjected to the low temperature sulphurizing treatment, the wear of the press-fit surface can be further reduced. This makes it possible to obtain the starter including the rotation transmitting mechanism having the torque absorbing mechanism which is further enhanced in durability.
According to the present invention, the above supporting shaft is a kick shaft. With this configuration, it is possible to obtain the kick-type starter capable of absorbing an excessive torque caused upon kick operation with a compact structure.
According to the present invention, the above supporting shaft is an idler shaft. With this configuration, since a rotatable gear set can be provided on an arbitrary shaft constituting part of the rotation transmitting mechanism, it is possible to increase the degree of freedom in design.
According to the present invention, one of the input gear and the output gear is press-fitted to a boss of the other of the input gear and the output gear, the other gear being different from the supporting shaft.
With this configuration, it is possible to broaden the ranges of selection of material characteristics such as the kinds of materials of the supporting shaft, the input gear, and the output gear.
According to the present invention, the above press-fitting is based on elastic deformation within an elastic limit. With this configuration, it is easy to set the fastening force at the press-fitted portion, and thereby the slip generating torque caused by press-fitting of the input gear and the output gear to each other.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.